I have exchanged my EL34 with the KT120 for testing purposes. Both run at 350V@55mA. For the EL34, a good operating Point as 350*0,55=19,25 Watt. For the KT120 I guess not optimal.

Ths question is if playing with the bias (easy) or the voltage (difficult), will Change the Sound in what way.

Currently the KT120 has a very good Body, a very good depth, but at the same time it Sound like a liitle curtain or minimal fog is between you and the musicians. While the EL34 is mor like you can touch them. The EL34 is nevertheless a Little less good organizes interms of who stands where, both left-right and front-back.

What transformer are you using Raa?? If the outputs are 5kRaa or higher, give the Russian 6P3S and 6P3S-E a try, better suited to 350V, but back off the bias. The 6P3S-E wafer base sound great and are low cost. The down side is that they cannot drive low impedance transformers, they run out of steam. For 4.3kRaa iron I use 6550 or Kt88's, I would recommend the Kt120's for 1.9Kraa applications with decent B+ 400 to 450V and run the heaters at 6.3V or a little hotter do not exceed the 6.9 limit. . A major design concern is over the input miller capacitance, the grid 1 capacitance difference between the EL34 and Kt120 are about double, 15pf to 29pf... The KT120s have to be driven to overcome that. The best substitute for EL34's that I like are the Svetlana 6550=C= tubes, best clarity and sound stage, better sound with less feedback needed.